It has been a conventional practice in the design of loaders, backhoes and similar machines to mount a material handling element, such as a bucket or boom, pivotally at one end of a base member, such as a lift arm or swing tower, with the hydraulic actuator or cylinder used to rotate the material handling element relative to the base member disposed above the base member. U.S. Pat. Nos. 4,053,075; 3,624,785; 2,645,369 and 3,447,708 are typical examples. Since one end of the hydraulic actuator is pivotally connected to the base member and the other end is pivotally connected to the material handling element, the hydraulic actuator extends in pivoting the material handling element away from the base member. Consequently, the cylinder end or head side of the piston within the actuator is pressurized in lowering the boom of a backhoe or in dumping the bucket of a loader. Similarly, the piston rod side of the piston is pressurized in raising the boom or lifting the bucket to its filled position. Since the piston rod side of the piston has an effective area less than the cylinder head side of the piston, and since the same hydraulic pressure is applied to each side of the piston, the force provided by the hydraulic actuator in lowering the boom of a backhoe is greater than the force provided in raising the boom. Similarly, in the case of a loader, the force provided by the hydraulic actuator in dumping the bucket is greater than the force provided in rolling back the bucket from its dumped to its filled position.
Since the piston rod portion of a hydraulic actuator occupies a certain fraction of the volume of the cylinder on the piston rod side of the piston, it takes a greater volume of fluid to extend the actuator than it takes to retract the actuator. Thus, the speed at which the boom of a backhoe is raised is potentially greater than the speed at which the boom is lowered. Similarly, the speed at which the bucket of a loader is rolled back relative to the lift arm, is potentially greater than the speed at which the bucket is dumped.
From the foregoing it should be clear that the conventional method of mounting hydraulic actuators on backhoes and loaders is just the opposite of what it should be. In other words the greater force capability of a hydraulic actuator should be used in raising the boom of a backhoe or in rolling back the bucket of a loader to its filled position. Conversely, when the boom is lowered or the bucket is dumped, the greatest available lifting force is not needed. All things being equal, the speed at which the boom is lowered and the speed at which the bucket is dumped should be greater than the speed at which the boom is raised or is dumped since the force of gravity is assisting the actuator. Thus, the conventional method of mounting hydraulic actuators on backhoes and loaders does not use the inherent capabilities of hydraulic actuators to the optimum extent.
Turning for the present to a backhoe, a backhoe implement is usually attached to the rear end of a tractor or similar machine in such a manner that the tractor operator or driver has to reverse his position on the tractor to operate the backhoe. Thus, the tractor driver faces the rear end of the tractor when the controls of the backhoe are operated. Since the bucket portion of the backhoe is pivoted towards the dipper stick when it is being filled, and since the dipper stick is pivoted to fold towards the boom of the backhoe when the loaded bucket is raised, the boom effectively obscures the tractor operator's view of the dipper stick, and more significantly, the inside of the bucket. In order to provide adequate lift capability and balance, it has been conventional practice to provide two hydraulic actuators on either side of the boom with the piston rod ends pivotally connected to the boom and the cylinder ends or cylinders pivotally connected to the swing tower. Thus, for the most part, the tractor or backhoe operator's direct line of sight of the bucket is shielded or obscured by the boom and the two hydraulic actuators used to operate the boom. In one machine, the overall width of the backhoe boom is about six inches and the overall width of each of the two hydraulic actuators used to operate the boom is on the order of 4.5 inches. Thus, a barrier of approximately 15 inches is interposed between the backhoe operator and the bucket.
Those skilled in the art known that there are many occasions during the operation of a backhoe when the backhoe operator must have an unobscured view of the bucket. This is particularly true when the bucket is being manipulated at close quarters to another worker or helper or when the backhoe bucket is being used close to formwork or scaffolding which could be easily damaged due to inadvertent or uncontrolled movement of the bucket, boom or dipper stick.
The conventional practice of flanking the boom by two hydraulic actuators or cylinders is especially troublesome when the backhoe is used to dig a narrow, deep trench. This is because the hydraulic actuators for the backhoe boom are relatively weak compared to the boom itself. This is especially true when the hydraulic actuators are fully extended, as would be the case when the boom is lowered inside a trench and a lateral force is imposed on the hydraulic actuator. This is an inherent weakness of all hydraulic actuators when they are fully extended. Hydraulic actuators, because they are designed to produce force in a direction of their longtitudinal axis, are inherently weak or provide relatively little resistance to severe lateral forces when they are fully extended. The only resistance against a force perpendicular to the longitudinal axis of the hydraulic actuator is that provided by the seals or rings on the piston and those seals between the piston rod and the cylinder. These seals, of course, are not intended to resist the lateral motion of the piston rod to any great extent. Comparatively speaking, the boom is a relatively strong structure and can easily resist forces or thrusts imposed laterally on the boom. Thus, if the boom is flanked by two hydraulic actuators it cannot be used to protect those actuators from lateral forces such as those which would be expected when the boom is driven against the walls of a deep, narrow trench.
In the case of a loader or bucket-loader, the conventional method of mounting a hydraulic actuator such that it is fully extended when the bucket is positioned for filling also subjects the hydraulic actuator to lateral forces just when it is least able to absorb or resist those forces. Those skilled in the art know that when a bucket is being filled the tractor operator frequently must "jockey" the position of the bucket and the tractor so that the bucket is completely filled when the bucket is "rolled back". While the lift arms are relatively rigid, the hydraulic actuator nevertheless is subjected to unbalanced lateral forces when the bucket is being filled. On the other hand, once the bucket has been raised to its filled position the bucket remains relatively stationary and static relative to the lift arms as the tractor is driven to another location. However, according to conventional practice, this is exactly when the hydraulic actuator is or has its greatest rigidity (i.e., it is contracted). Thus, even in the case of a loader, the conventional method of mounting hydraulic actuators not only fails to use the inherent force-producing capabilities of the hydraulic actuator but also subjects the hydraulic actuator to dynamic loadings at a time when it is least able to resist lateral forces.
Hydraulic actuators of the type used in conjunction with earth working implements such as loaders and backhoes are also vulnerable to damage in that the polished piston rods emerging or extending from the cylinder portion of the actuator are exposed to damage from falling rocks and other debris which may occasionally spill over the rear edge of the bucket of a loader or from the walls of the trench or excavation in which the backhoe is inserted. If the hydraulic actuators are so mounted that they are in their contracted position when they are most likely to have dirt or other debris dumped upon them, the polished piston rods will be protected and the seal between the piston rod and the cylinder will be less likely to be damaged. U.S. Pat. No. 3,197,050 describes this consideration in reference to a loader mechanism. However, even where the principle was recognized, the linkage was relatively complicated and employed a large number of pivotal connections and relatively expensive trunion mounted hydraulic actuators. A simplified linkage which incorporates the inherent capabilities of a hydraulic actuator has heretofore eluded those skilled in the art.
Thus, it should be appreciated that a backhoe design or loader design which would use the inherent force and speed capabilities of a hydraulic actuator to manipulate a boom or a bucket and which would mount the actuator in such a manner that it would be best able to absorb side thrusts or lateral forces, would not only improve the overall productivity of the machine but would also reduce maintenance and repair costs. A design change that would achieve these benefits, while at the same time reducing the overall cost of producing the backhoe or loader mechanism would be well accepted by the industry and would go far towards achieving an optimum machine design.